Letter Cite This: Nano Lett. 2018, 18, 5426−5431
pubs.acs.org/NanoLett
Carbon Nanotube Chirality Determines Properties of Encapsulated Linear Carbon Chain Sebastian Heeg,† Lei Shi,‡ Lisa V. Poulikakos,§ Thomas Pichler,‡ and Lukas Novotny*,† †
ETH Zürich, Photonics Laboratory, 8093 Zürich, Switzerland University of Vienna, Faculty of Physics, 1090 Wien, Austria § ETH Zürich, Optical Materials Engineering Laboratory, 8093 Zürich, Switzerland ‡
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ABSTRACT: Long linear carbon chains (LLCCs) encapsulated inside double-walled carbon nanotubes (DWCNTs) are regarded as a promising realization of carbyne, the truly onedimensional allotrope of carbon. While the electronic and vibronic properties of the encapsulated LLCC are expected to be influenced by its nanotube host, this dependence has not been investigated experimentally so far. Here we bridge this gap by studying individual LLCCs encapsulated in DWCNTs with tip-enhanced Raman scattering (TERS). We reveal that the nanotube host, characterized by its chirality, determines the vibronic and electronic properties of the encapsulated LLCC. By choice of chirality, the fundamental Raman mode (C-mode) of the chain is tunable by ∼95 cm−1 and its band gap by ∼0.6 eV, suggesting this one-dimensional hybrid system to be a promising building block for nanoscale optoelectronics. No length dependence of the chain’s C-mode frequency is evident, making LLCCs a close to perfect representation of carbyne. KEYWORDS: Linear carbon chains, carbyne, carbon nanotubes, Raman spectroscopy, TERS
C
transfer, or dielectric screening and may even limit the chain length.10−20 These interactions vary with chirality, modify the BLA, and effectively mask the intrinsic properties of LLCCs.22,23 Recently, it was even suggested that there is a residual length dependence of the chain’s properties far beyond the currently accepted limit of 100 atoms.17,18,22 This would mean that LLCCs are not the finite realization of carbyne and that the currently accepted models on the chain’s length dependence fail. However, while the properties of confined long linear carbon chains are governed by their nanotube host and potentially by their length, the correspondence between the host tube’s chirality, the properties of the encapsulated chain, and the length of the chain have not been investigated experimentally. Raman spectroscopy is an excellent tool to study the properties of the encapsulated chain and the characteristics of the encasing CNT. The dominant Raman mode of carbyne (Cmode) reports the bond-length alternation of the chain.1 The chirality-specific Raman features of CNTs, in particular the radial breathing mode (RBM), are very well understood.24,25 Accordingly, Raman measurements of nanotubes hosting LLCCs allow us to correlate the electronic and vibronic
arbyne by definition is an infinitely long linear carbon chain (LLCC) with sp1 hybridization that forms the truly one-dimensional allotrope of carbon at the one-atom crosssection limit.1−3 Its anticipated stiffness, strength, and elastic modulus exceed that of any other known material.4 In its most common form, carbyne is a polyyne with alternating single and triple bonds originating from a Peierls distortion.5 This bondlength alternation (BLA) dominates the electronic and vibronic structure of carbyne.6−8 It opens up a direct band gap that is sensitive to external perturbations, thus offering tunability. Finite linear carbon chains are expected to exhibit the properties of carbyne if they consist of 100 or more atoms as the BLA saturates.1 Exploring the fundamental properties of carbyne experimentally, however, has long been hindered by its extreme chemical instability and short chain lengths of up to 44 atoms.9 The synthesis of linear carbon chains inside carbon nanotubes, sketched in Figure 1(a), overcomes these obstacles.10−16 The tubes act as nanoreactors, prevent chemical interaction of the chains with the environment, and allow for long chain lengths. A major leap forward in forming carbyne are long linear carbon chains with lengths up to several hundreds of nanometers that have recently been synthesized inside double-walled carbon nanotubes (DWCNTs).17−21 The local environment inside a nanotube, characterized by its chirality, is expected to affect encapsulated chains through interactions such as van-der-Waals (vdW) forces, charge © 2018 American Chemical Society
Received: April 25, 2018 Revised: August 2, 2018 Published: August 8, 2018 5426
DOI: 10.1021/acs.nanolett.8b01681 Nano Lett. 2018, 18, 5426−5431
Letter
Nano Letters
transmit the spectral region of the linear carbon chain’s Cmode (1770 cm−1 to 1870 cm−1)10−21 followed by a singlephoton counting avalanche detector or by a combination of spectrograph and charge-coupled device (resolution ∼3 cm−1). To avoid sample degradation due to heating, we used laser powers of 115 μW for TERS and 1.15 mW for confocal (CF) imaging. Integration times are 25 ms/pixel for imaging and up to 60 s for acquiring full Raman spectra. LLCCs were grown inside DWCNTs in a high-temperature and high-vacuum process as described in ref 17. The tubes were then purified, individualized, and dispersed on thin glass cover slides using chlorosulfonic acid in an oxygen-free atmosphere. To locate individual carbon chains inside DWCNTs we first perform confocal Raman raster scans (not shown). Once the signature of a linear carbon chain is detected, we verify its presence by observing the C-mode of the chain in a confocal Raman spectrum (black) as shown in Figure 1(b). We then position the tip near the surface (
